1. Field of the Invention
The present invention relates to foam bonded pre-insulated piping systems, and more specifically to a structure and method for preventing the disbondment of the foam from the carrier pipe as these systems thermally expand in the presence of high temperature fluids being conveyed.
2. Description of the Prior Art
There are many instances in which insulated pipelines are needed. For example, distributed HVAC (heating, ventilation and air conditioning) applications utilize chilled water for cooling and steam for heating. The chiller and boiler are typically contained in a central location and the chilled water and steam are distributed to other locations. For example, on a school campus, the chiller and boiler may be located in a power plant building. The chilled water and steam are distributed to classrooms in separate buildings.
A set of insulated pipelines is used to convey the chilled water from the chiller to other locations and back to the chiller. Another set of insulated pipelines is used to carry the steam from the boiler to the other locations and back to the boiler. The insulated pipelines are usually located underground.
Insulated pipe is conventional and commercially available. There are predominately two types of piping systems in use: Class-A drainable dryable testable (DDT); and polyurethane or polyisocyanurate bonded foam systems. The present application is directed toward the bonded foam type system. These systems utilize a steel pipe to convey fluid. Around the outside of the steel pipe is a layer of insulating foam such as, for example, polyisocyanurate foam. Around the outside of the foam is a jacket of hard thermoplastic (such as high density polyethylene, HDPE). The plastic jacket protects the foam from mechanical damage and also provides a water tight seal to prevent corrosion of the steel pipe. Although steel is commonly used for the inner pipe which carries the media to be piped, copper or aluminum or other metals as well as fiberglass, PVC, and similar materials may be utilized, as well.
The most important engineering criteria for a foam system of the type under consideration is that it must be treated as a bonded system. In other words, the foam is bonded to both the carrier pipe and the outer jacket. Therefore, the bonded system acts as a monolithic unit moving underground. Higher temperatures can act adversely upon the bonded foam system, however. The hot fluid in the steel carrier pipe causes the carrier pipe to thermally expand. At temperatures of 400° F. this expansion is on the order of 2.8 inches per 100 feet of pipe. This expansion is not a problem as long as the system remains bonded and the carrier pipe, foam and jacket move together as one unit underground. This movement is controlled by the expansion force of the steel carrier pipe, but it is the bond strength of the foam to the pipe and jacket that is important in keeping the system moving together. This monolithic movement of the system occurs along each incremental length of a particular run, and as long as total movement is not greater than 4 to 6 inches and the system remains bonded, no undue stress is subjected at any one point of the jacket. If the system however were to disbond, the surrounding soil would fix the jacket in place and the carrier pipe would still thermally expand thereby pushing thorough and destroying the jacket at the first change of direction.
It has been well established by industry case history that the polyurethane foam bond for systems running at 250° to 300° is strong enough to keep the entire system acting as a bonded system. However, for systems running above these temperatures a higher temperature rated foam, such as polyisocyanurate foam, is required. Even in systems utilizing “high temperature” polyiscyanurate foam, the higher heat can begin to fry the foam at the foam/carrier pipe interface thereby bringing into question the strength of the foam bond to the steel carrier pipe. Therefore, systems with temperatures exceeding 250° to 300° F. are at risk of having the carrier pipe disbond and push through the jacket at changes of direction.
Various approaches have been taken to control this undesirable expansion in insulated pipe systems of the type under consideration. For example, expansion “bolster” materials are supplied in the form of resilient pads which can be used to wrap the HDPE jacket at elbows or expansion loops. Various mechanical approaches have also been tried including bellows-like structures along the length of the pipe line or at the elbows or expansion loops.
Despite the above advances, a need exists for improvements in a bonded foam system which will ensure that the insulating foam remains bonded to the carrier pipe. The advent of higher temperature foams allows for higher temperature foamed piping systems to be manufactured. However, at temperatures exceeding 250° to 300° the bond strength of the newer foams comes into question.
A need continues to exist for a bonded foam system of the above type which effectively prevents or eliminates foam disbondment, even at temperatures above 250 to 300° F.